Hardening bath



Patented Dec. 2, 1 952 UNITED STATES PATENT OFFICE HARDENING BATH No Drawing. Application October 2, 1950, Serial No. 188,069

Claims.

It is the general object of my invention to provide an improved hardening method and molten salt bath of the conventional type in which the decarburization is reduced to a minimum or altogether prevented.

Another object of my invention is to provide an improved hardening bath characterized by the addition of a small quantity of magnesium fluoride to the molten bath.

A further object of my invention is to provide an improved alkali metal chloride bath which is characterized by the addition of 1 to 50, preferably 2 to 5 parts of magnesium fluoride to the salt bath.

With prior art processes the well known hardening of tools is carried out by a heat treatment of the tool steel at a suitable temperature and subsequent quenching. In order to minimize or completely avoid a harmful decarburization of the tools which causes a soft layer and subsequently a deficient hardness of the tools, it is necessary to add substances which are able to prevent decarburization.

Such substances are, for instance, the alkali or alkaline earth cyanides for use at hardening temperatures up to 950 C.; furthermore, borax, acid oxides, for instance the oxides of titanium, silicium and the like for use at hardening temperatures up to 1300 C. and more.

Whilst it is possible to substantially reduce the decarburization by adding cyanides at a temperature of about 950 0., it still involves considerable difficulties to prevent decarburization at higher temperatures if boric acid, silicic acid or the like are used at higher temperatures. Insufficient quantities of these additional substances result in decarburization and too high quantities of these substances result in objectionable corrosion. Nearly always a harmful formation of a viscous slag was observed.

The invention is based on the discovery that it is possible to prevent any harmful decarburization in adding magnesium fluoride'to the molten salt bath. The quantity of the added magnesium fluoride may vary in wide ranges according to the composition of the molten salt bath, for instance, between 1 and 50 percent, based on the weight of the salt bath. In general, quantities up to percent are already sufiicient. Excellent results were obtained with quantities of 2 to 5 percent of magnesium fluoride.

The following specific examples are given as an illustration of the technical effect of my invention:

1. A steel sheet of 0.5 mm. thickness with a carbon content of 1.3 percent is heated for a period of 5 minutes in a conventional barium chloride bath at a temperature of about 1300 C., the bath containing 1 percent of an acid barium silicate to prevent decarburization. After 5 minutes soaking the carbon content of the sheet dropped to 0.7 percent. Investigation had led to the knowledge that hitherto the said degree of decarburization was nearly always observed in the conventional salt baths, although, the aforementioned degree of decarburization is decidedly harmful in many cases.

2. In comparison with this test the same sheet was dipped for a period of 5 minutes into a molten barium chloride bath which in accordance with this invention also contained an addition of 4 percent of magnesium fluoride. After 5 minutes soaking the carbon content of the steel sheet was still 1.3 percent. As a result the decarburization was altogether prevented.

Contrary to the indications in the prior art sodium fluoride as well as calcium fluoride have no preventive effect on decarburization as illustrated in the following example:

3. A steel sheet of 0.5 mm. thickness was dipped into a molten salt bath which contained barium chloride with an addition of 5.6 percent of sodium fluoride. After a period of 5 minutes the carbon content of the sheet dropped to 0.8 percent.

Further investigations have led to the knowledge that calcium fluoride as an addition to a conventional molten salt bath has no reducing effect on decarburization.

In accordance with the invention it was found that in adding magnesium fluoride to a salt bath no viscous or slow fusible compounds are formed. This contrasts with the use of boric acid, acid silicates and the like. It was found that the bath remains fluid and that no layer is formed on the surface of the bath. The optical measuring of the temperature may therefore easily be carried out in the salt bath.

According to my invention it has proved advantageous to use molten salt baths which are substantially free from carbonates such as, for instance, sodium carbonate, barium carbonate and the like. The action of magnesium fluoride is especially remarkable if the bath is kept in operation for a long period of time. The following table illustrates the efiect of decarburization in a salt bath consisting only of barium chloride, and a bath consisting of barium chloride and 4 percent of magnesium fluoride, and a bath consisting of barium chloride and 5.6 percent of sodium fluoride.

3 The baths were kept in operation for 72 hours and as a rule of practice, one tenth of the contents of the bath was substituted by a fresh mixture of the same composition at regular intervals of 8 hours.

chloride selected from the group consisting of alkali and alkaline earth metal chlorides and 1% to 50% by weight of magnesium fluoride.

2. A salt bath for the heat treatment of steel at temperatures above 1000 C. consisting of 50% Table hours 8 16 24 32 40 48 5G 64 72 Composition of the bath: Percent C Percent 0 Percent C Percent'O Percent 0 Percent 0 Percent C Percent C Percent C B8013 0. 85 0. 84 0. 87 0.57 0.87 0. 49 0. 67 0. 23 Bac1z+4% MgFz 1. 20 1. 32 1. 33 1. 29 1. 21 1. 22 1. 20 1. 23 1. 21 Bach-H.67 NaF 1.00 1.10 0.92 0.87 0,47 0.36 0.39 0.14 0.

In accordance with the invention the table 15 to 99% by weight of barium chloride and 1% to indicates with complete certainty the superior effectiveness of the addition of magnesium fluoride to a molten salt bath of the alkali metal or alkaline earth metal type.

By following the teachings of this specification, molten salt baths with the addition of magnesium fluoride may also be used at lower temperatures, such as temperatures below 1000 C. Good results were obtained, for instance, with a bath consisting in 50 parts of sodium chloride, 48 parts of potassium chloride and 2 parts of magnesium fluoride.

Although I have particularly described the principle of my process and preferred ways of practicing the process, and the particular and specific mixtures of substances preferred in practicing the process, nevertheless, I desire to have it understood that the examples given are merely illustrative, but do not exhaust the possibilities of the process, or the possible substances usable in carrying out the process.

What I claim is:

l. A salt bath for the heat treatment of steel comprising 50% to 99% by weight of at least one CARL ALBRECHT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,046,822 Holden July 7, 1936 2,394,777 Holden et a1 Feb. 12, 1946 2,400,511 Holden May 21, 1946 

1. A SALT BATH FOR THE HEAT TREATMENT OF STEEL COMPRISING 50% TO 99% BY WEIGHT OF AT LEAST ONE CHLORIDE SELECTED FROM THE GROUP CONSISTING OF ALKALI AND ALKALINE EARTH METAL CHLORIDES AND 1% TO 50% BY WEIGHT OF MAGNESIUM FLUORIDE. 